The present invention concerns a process for the production of at least one alkyl tertiobutyl ether at least partially from natural gas.
Natural gas is an abundant raw fossil material mainly constituted by methane, whose current known reserves, of the order of 10.sup.14 m.sup.3, represent about 50 years of world consumption. The gas fields frequently contain large quantities of ethane, propane, other high alkanes (C.sub.3.sup.+ hydrocarbons, i.e., hydrocarbons containing at least 3 carbon atoms per molecule), as well as other constituents such as H.sub.2 O, CO.sub.2, N.sub.2, H.sub.2 S and He. The majority of the propane and other high alkanes associated with natural gas are liquefied as LPG (liquid petroleum gas). In helium rich fields (generally more than 0.3% by volume), helium is separated out because of its high commercial value. Hydrogen sulphide is also separated out because of its corrosive nature, and also water to prevent the formation of hydrates which make the transport of natural gas difficult. Natural gas can be purified by dehydration and desulphurisation, preferably to a sulphur compound content of less than 10 ppm. It is then termed uncondensed gas and contains mainly methane (for example, 55-99% by volume), also ethane, usually propane and may contain small amounts of nitrogen and/or carbon dioxide.
However, the term natural gas is indiscriminately applied to purified gas obtained by dehydration and desulphurisation, and to unpurified gas obtained directly from a field and not subjected to any purification whatsoever.
Most natural gas is used for private and industrial heating. However, some processes exist for transforming natural gas into higher hydrocarbons.
Transformation of natural gas into alkyl tertiobutyl ether(s), such as methyl tertiobutyl ether (MTBE) or ethyl tertiobutyl ether (ETBE) respectively, is highly desirable. Alkyl tertiobutyl ethers are currently produced by reacting isobutene with an alcohol, such as methanol or ethanol respectively, and while said alcohols are readily synthesized from natural gas via synthesis gas (defined as a mixture containing carbon monoxide, hydrogen and, optionally, carbon dioxide), this is not the case for isobutene which is normally extracted from a C.sub.4 (hydrocarbons containing 4 carbon atoms per molecule) cut, produced from a catalytic cracking unit for petroleum cuts, for example. This source of olefins is now far too limited to fulfil the increasing demand for alkyl tertiobutyl ether(s) such as MTBE or ETBE. For this reason, a method of producing alkyl tertiobutyl ethers from natural gas would be of interest.